The present invention relates to a method of and an apparatus for actuating a vehicle transmission for use in an automobile.
Vehicles, typically automobiles, powered by internal combustion engines have a transmission coupled between the engine and driven wheels for effective utilization of output power from the engine. In operation, one of the different gear ratios of the transmission is selected to meet the particular vehicle speed. For example, when the vehicle is to run at a low speed, a larger gear ratio is used to cause the engine to operate at a high speed.
Several transmissions are known in the art. One known type is a parallel-gear transmission comprising a plurality of parallel gears, some of which are axially movable for changing gear ratios, and an internal level for shifting different gears into meshing engagement. For a transmission having five-speed and one-reverse gear positions, the internal lever is shiftable between three neutral positions N1, N2 and N3 in a row in a Y direction, between 1st and 2nd positions in an X direction normal to the Y direction across the neutral position N1, between 3rd and 4th positions in the X direction across the neutral position N2, and between the 5th and reverse positions in the X direction across the neutral position N3. Therefore, the internal lever is movable along the path composed of two "H"s partially overlapping each other.
There has recently been put to use a transmission system having a hydraulic drive mechanism controlled by an electronic control unit for automatically driving the internal lever. The transmission system has a select actuator for driving the internal lever selectively into the neutral position N1, N2 and N3 in the Y direction, and a shift actuator for driving the internal level selectively into the 1st through 5th and reverse positions in the X direction. When the transmission gear is to be changed from the 2nd position to the 3rd position, the electronic control unit operates automatically to enable the shift actuator to move the internal lever in the X direction into the neutral position N1, then cause the select actuator to move the internal lever from the neutral position N1 to the neutral position N2 in the Y direction, and finally enable the shift actuator to move the internal lever from the neutral position N2 to the 3rd position in the X direction. Each of the actuators for automatically driving the internal lever comprises a hydraulic cylinder and a piston slidably disposed therein and movable into three positions, that is two end positions and one intermediate position, in response to a hydraulic pressure introduced into cylinder chambers defined one on each side of the piston. For moving the piston to the right, the hydraulic pressure is supplied into the cylinder chamber on the lefthand side of the piston, while the fluid in the righthand cylinder chamber is forced by the piston to return through a discharge passage into a tank. To permit the transmission to operate smoothly and prevent members of the transmission such as a shift fork from being worn quickly, the actuators will be released of any hydraulic pressure after the gear changing operation has been finished. The discharge passages from the actuators are connected in common. In the case where an automatic clutch is incorporated, the discharge passage of its actuator is also coupled in common to the discharge actuators of the transmission actuators. One disadvantage with such an automatic transmission system is that while the internal lever is being shifted by the shift actuator from one of the gear positions to the neutral position, the hydraulic pressure in the discharge passages is varied by the fluid discharged from the shift actuator, and such pressure variation tends to act on the select actuator and actuate the latter. This impairs smooth operation of the shift actuator since the select actuator is liable to apply forces to the internal lever perpendicular to the direction in which the internal lever is shifted by the shift actuator. The above shortcoming could be avoided by providing the select and shift actuators with independent discharge passages. However, this proposal is disadvantageous in that the entire fluid discharge piping would become complex.